Vitreous enameling process and composition



2,702,762 Patented Feb. 22, 1955 VITREOUS ENAIVIELING PROCESS AND COMPOSITION Application September 9, 1953, Serial No. 379,287

11 Claims. (Cl. 11753) No Drawing.

This invention relates to vitreous enameling, more particularly to new and improved base stock for vitreous enamels and to a new and improved method of controlling the adherence of vitreous enamels to ferrous metals.

The present application is a division of my copending application U. S. Serial No. 161,248, filed May 10, 1950.

As is well known, one of the principal problems in the art of vitreous enameling is to secure proper adherence between the vitreous enamel coating and the base stock to which the enamel is applied, without the use of a ground or grip coat containing cobalt, manganese and usually nickel. It has long been recognized that it would be desirable to employ as the enamel base stock ordinary carbon steels containing the usual percentage of carbon (e. g., 0.10% to 0.20%), but such steels are characterized by rather poor warping properties as compared with the special enamel irons or enamel steels when exposed to heat during the burning operation. As a result, the most satisfactory vitreous enamel base stocks have been specially prepared low carbon steels (containing, say, 0.01% to 0.02% carbon) which are more expensive than many of the ordinary steels.

One of the objects of this invention is to provide a new and improved method of controlling the adherence of vitreous enamels to ferrous metal base stocks.

Another object of the invention is to provide a new and improved method of controlling the oxidation and reduction reactions which occur during the firing of a vitreous enamel.

A further object of the invention is to provide a new and improved type of material which is specially adapted to receive a coating of a vitreous enamel.

Generally stated, the invention relates to a new step in the process of enamel coating ferrous metals which consists of applying to the surface of the metal a bonding composition having as one of its essential ingredients an antimony compound which is preferably dispersed in an inert dispersion medium containing finely divided, preferably colloidal, particles. The invention also contemplates the addition to the above described ingredients of a substantial portion of a chloride from the group consisting of barium, strontium, manganese and magnesium chlorides. The bonding materials disclosed herein possess special utility as a spray to provide a thin evenly dispersed primer coat on the ferrous metal surface, said primer coat insuring complete adherence of the subsequently applied enamel color coat. The antimony compound and chloride in the dispersion medium are evenly dispersed over the surface of the metal so that minute particles thereof are presented to the metal surface. The enamel frit or coat can be applied directly to the coated metal surface while still wet after which the firing of the enamel coated metal sheet may be carried out without intermediate drying.

The vitreous enamel firing temperatures will vary, depending upon the composition of the vitreous enamel. For example, a high firing vitreous enamel may be fired at temperatures as high as 1650 degrees F. while a low firing vitreous enamel may be fired at temperatures around 1250 degrees F. or below. The invention especially contemplates a vitreous enameling method which makes it possible to employ low firing vitreous enameling compositions which can be fired at much lower temperatures than heretofore considered feasible. The minimum firing temperature will naturally vary, depending upon the type of vitreous enameling composition, but vitreous enameling compositions firing at temperatures as low as 800 degrees F. or lower are contemplated in accordance with the invention.

Example Tartar emetic was dissolved with barium chloride in water in the proportion of 7 parts by weight of tartar emetic, 7 parts by weight barium chloride, and 86 parts by weight water. The solution was subjected to thorough agitation and then mixed with commercial sodium silicate m a ratio of 50 parts of said solution to 50 parts of sodium silicate. This resulted in the formation of a silica gel by the reaction of the sodium silicate with the tartaric acid radical of the tartar emetic.

The resultant composition togetherwith the supernatant hquor was passed into a paste mill and then mixed with a bentonite suspension in the proportion of 28% parts by weight to 71 parts by weight of the bentonite suspension. The bentonite suspension was prepared by mixmg 3, parts by weight of bentonite with 97 parts by weight of water with heating and agitation.

The foregoing composition was then introduced into a ball mill where it was ground to a superfineness so that the resulting thin colloidal suspension could be sprayed in a very thin film on a previously pickled metal sheet or article.

The bonding composition as thus prepared, when applied as a spray to the ferrous surface to be coated, will uniformly disperse over the surface of the metal, the antimony acting as a reducing agent to reduce the valence of the lIOIl in the thermally formed iron oxide while the chloride phase of the composition functions to increase the oxidation rate, particularly at the fusion temperature of medium and low firing enamels, thereby providing for a strong adherence of the vitreous body to the ferrous metal surface. The bentonite and silica gel function as a vehicle to hold the compound in a finely divided condi-. tron suitable for use in a spray gun, whereby the surface to be enameled can be sprayed evenly with the compound to prepare the metal surface immediately, without drying, for the application of the enamel color-coat to be fired thereon.

Tartar emetic, which is also known technically as potassium antimonyl tartrate, is the preferred antimony compound for the practice of the invention because it is a salt of an organic acid which will react with an alkaline silicate to form a silica gel as described in the previous example. It is also substantially water soluble and therefore readily dispersed in an aqueous dispersion medium. Other antimony compounds, such as potassium pyroantimonate and antimony oxide, can be employed for the purpose of the invention but are less desirable in that they do not have all of the aforementioned properties of tartar emetic. A coating of antimony trichloride on steel tends to explode under the impact of a sharp blow. Hence, it 1s preferable to employ the antimony and chlorine in separate components of the composition.

The metal chloride should preferably contain at least two atoms of chlorine per molecule so that the propornon of chlorine to metal is relatively high. Otherwise, large quantities of unnecessary elements are present in the coating composition and too great a bulk of material 1s required to supply the necessary chlorine. Examples of suitable chlorides are barium chloride, strontium chloride, manganese chloride and magnesium chloride. The chloride should also be substantially neutral for the best results. Acidic chlorides may be used where they are neutralized by the alkaline silicate.

An example of glass or frit which can be used with the bonding method is as follows:

This material is mixed, smelted, fritted and dried in the usual manner, and to produce a light grey coating the ll addition o he above w l be as. follows:

Frit parts by weight 100 Clay parts by frit weight 7 Opacifier do 3 Illmenite do Black oxide iron do 36 Borax do Water do 40 This material is ground or milled to a fineness of O25% residue on a 200 mesh screen, preferably 48% residue, and the metal as previously sprayed with the new bonding compound described is then coated with this milled slip by spraying. Finally the coated piece is dried and fired' at a temperature below 158i) degrees F., e. g., in a range between 110i) degrees F. to 1580 degrees F.

An important advantage of the invention lies in the simplicity with which the process may be carried out. The bonding composition is simply and inexpensively prepared into a colloidal suspension which is particularly useful a a ra e a pl aat aa of h n w b n n qqmp p is b p P o s hi h 9$ t e u d in efo h pp q t n t t e enamel r tl he advantages result ins lessening of the cost of application of the enamel coating, while at the same time providingYad herence which is as good or better than that obtained bythe use of a conventional vitreous enamel r n at.

The invention is hereby claimed as follows:

1. In a method of coating ferrous metals with vitreous enamels, the step which comprises pretreating the ferrous metalwith a substantially uniform thin coating containing potassium antimonyl tartrate and a chloride of a metal m the r 'c n st of a um. tr n um, ma ganese andrnagnesium.

2- In a m od of co term m ta w h v t u enamels, the step which comprises pretreating the ferrous metal with a substantially uniform thin coating containing tartar emetic and barium chloride,

In ii P QQ S o a qg ferrous metals, with vitreous. naml he te ic cam uses pr ea n h terqu m t a sub a ial uni orm t n co t ng uain' ng a ar e a a bar um. hlo e nd pendin agent.

4; The method of coating ferrous metals with vitreous n mels hi h ris a -i s tar a emetic intimat ly dispersedwith barium chloride to the metal surface,

h r af e pp y a ow ri enamel CQ I PQsitiQI a a le of b ls' a e ed 0 a i au enamel by firin t m a m t in h n e o 9 de ees F. o 1580 degrees F., and then firing said enarnel composition.

5. A bonding composition for pretreating ferrous metals 9 n reoi amels th re qon st 's e sen a y of a di i l n r di BQtSi l mdiuzfi of pat s i m mi l nr ni a e and. a. ubs ant ally n utra ch o d o 4' a metal from the group consisting of barium, strontium, man anese andmasnasium 6. A bonding composition for pretreating ferrous metals I to bond vitreous enamels thereto consisting essentially of a dispersion in an inert aqueous dispersion medium comprising silica gel together with potassium antimonyl tartrate and a substantially neutral chloride of a metal from the group consisting of barium, strontium, man- 10 ganese and magnesium.

coating ferrous metals composed of tartar emetic, barium chloride, sodium silicate and a suspending agent.

8. A bonding composition for proces'sesof enamelcoating ferrous metals compqsedof tartar emetic, barium chloride, sodium silicate and bentonite,

9. A bonding composition for pretreating ferrous metals to bond vitreous enamels theretowhich comprises tartar emetic, barium chloride, and an inert dispersion medium therefor, said dispersion medium being effective to disperse said tartar emetic and said chloride over the surface of said ferrous metal.

A ban n omposi on o p re t ng. ferr s m a to bon i eou ena el th o hiQll ampris ta m ic barium ahl rid and a di pe s n mqdium fe t ve to i e se s id. t tar'e s s Over the sur a of said ferrous metal, said dispersion medium comprising silica gel.

P o f p p r n a o d n ca pe it q f r use in enamel-coating processes which consists in preparw ing a solution of tartar emetic, chloride of barium and water, subiecting the said solution to agitation and then adding tothe mixture sodium silicate thereby forming silica gel as a part thereof, then subjecting the silica gel combination to the action of a paste mill followed by introducing into. said mixture an aqueous bentonite suspension to complete the composition, and finally grinding the complete composition to a fineness for use' as a spray.

40 References Cited in the file of this patent UNITED STATES PATENTS 1,568,669 Hull Jan. 5, 19,26 ,7 ,0 C0 6 em- .4 2 0 01 MqDqllald fib- 9.3 13 E 'iS b- 2,321,656 Chester June 15, 1943 0 2,321,658 Chester June 15, 1943 OTHER REFERENCES Manson: I. Am. Ceram. Soc. (1923), pp. 790-793. Tetrick: I. Ceram. Soc. (1934), pp. 349456. Smithells: Metals Reference Book, published (1949-) by Interscience Publishers Inc. New York, NewYork,

page 68 7. A bonding composition for processes of enamel- 

1. IN A METHOD OF COATING FERROUS METALS WITH VITROUS ENAMELS, THE STEP WHICH COMPRISES PRETREATING THE FERROUS METAL WITH A SUBSTANTIALLY UNIFORM THIN COATING CONTAINING POTASSIUM ANTIMONYL TARTRATE AND A CHLORIDE OF A METAL FROM THE GROUP CONSISTING OF BARIUM, STRONTIUM, MANGANESE AND MAGNESIUM. 